1. Field of the Invention
The present invention relates to a semiconductor photosensor, and more specifically to an avalanche photodiode of a low noise, low dark current, and high speed response characteristics.
2. Description of Related Art
To realize a high-speed, large-capacity optical transmission system, semiconductor photosensors of a low noise, low dark current, and high speed response characteristics are indispensable. Recently, for this purpose, a research for increasing the response speed and sensitivity of an InP/InGaAs avalanche photodiode which can be used in a low loss wavelength region of 1.3 .mu.m to 1.6 .mu.m of a silica optical fiber, is actively made. In this InP/InGaAs avalanche photodiode, a gain bandwidth product of 75 GHz has been realized by decreasing a receiving aperture diameter so as to reduce the capacitance, by optimizing the layer thickness for reducing the carrier transit time, and by introducing an intermediate layer at a heterojunction interface for controlling a carrier trap.
In the above mentioned InP/InGaAs avalanche photodiode, however, since the ionization rate ratio .alpha./.beta. (where .alpha. is an electron ionization rate and .beta. is a hole ionization rate) is as small as 2 or less, the excess noise factor "x" becomes as large as 0.7 or less (because, the smaller the ionization rate ratio is, the larger the excess noise factor is). Therefore, there is a limit in realizing the low noise and the high sensitivity. This can also applied in the case that the avalanche multiplication layer is formed of other bulk III-V group compound semiconductor. Therefore, for realizing the low noise and the high gain bandwidth product (high speed response), it is necessary to artificially increase the ionization rate ratio .alpha./.beta..
Thus, F. Capasso et al proposed, in Appl. Phys. Lett., Vol. 40(1), pp 38-40, 1982, a structure in that the ionization rate ratio .alpha./.beta. is artificially increased by utilizing a conduction bank energy discontinuity amount .DELTA.Ec in the superlattice for impact ionization of electrons. It was actually confirmed that the ionization rate ratio .alpha./.beta. is increased in GaAs/GaAlAs superlattice structure (.alpha./.beta. was increased to 8 at maximum in the superlattice, in comparison to 2 at maximum in a bulk GaAs). In addition, Kagawa et al reported, in Appl. Phys. Lett., Vol. 55(10), pp 993-995, 1989, that a similar structure was formed in an InGaAs/InAlAs superlattice having a light receiving sensitivity in a band of the wavelength 1.3 .mu.m to 1.6 .mu.m, which is used in a long distance light communication, and the increase of the ionization rate ratio .alpha./.beta. was similarly confirmed (.alpha./.beta. was increased to 10 at maximum in the superlattice, in comparison to 2 at maximum in a bulk InGaAs).
More specifically, the Kagawa et al structure, the avalanche multiplication layer is formed of an alternating repetition of n.sup.- In.sub.0.52 Al.sub.0.48 As barrier layers, and n.sup.- In.sub.0.53 Ga.sub.0.47 As well layers, which form a superlattice avalanche multiplication layer.
In this structure, a conduction band energy discontinuity amount .DELTA.Ec is 0.5eV which is larger than a valence band energy discontinuity amount .DELTA.Ev of 0.2eV. Therefore, when the carriers enter in the well, the energy acquired due to the band discontinuity is larger in the electron than in the hole, so that the electron becomes easy to reach an ionization threshold energy. Thus, the electron ionization rate is increased, and the ionization rate ratio .alpha./.beta. is correspondingly increased.
In the above mentioned structure of avalanche photodiode, however, a tunnel dark current occurring in the well layers (n.sup.- In.sub.0.53 Ga.sub.0.47 As well layer) having a short valence band width within the superlattice avalanche multiplication layer, extremely increases to a micro-ampere order, in a practical use region in which the multiplication factor is a few times or more.